Hepatocellular Carcinoma is a major health concern. Currently, this disease is the 3rd leading cause of cancer deaths worldwide with over 500,000 new diagnoses each year. This high incidence of hepatocellular carcinoma development is attributed to environmental insults ranging from exposure to hepatocarcinogens, viral infections, to metabolic disorders. One of the key genetic lesions found in liver cancer is inactivation of the retinoblastoma tumor suppressor (RB) pathway. This pathway is disrupted in most liver tumors via a number of discrete mechanisms including loss of RB gene or deregulation of protein phosphorylation (e.g., p16ink4a loss). Over the last funding cycle we extensively interrogated how RB loss contributes to tumor development. In mouse models, we found that in the absence of carcinogenic stress RB loss is insufficient for tumor development. This is due to extensive compensation of the RB-deficient state and led us to explore unique functions of RB that are not subject to compensatory processes. These analyses demonstrated a novel function of RB in maintaining transcriptional control and cell cycle exit in the presence of specific carcinogenic stresses. Consonantly, RB-deficiency was associated with significantly increased tumor development under such conditions. These combined findings provide the basis for fully understanding how RB-status impinges on transcriptional processes under stress conditions and the specific consequence related to genome integrity driving tumor development (Aim 1). Interestingly, in the course of these studies we found that RB loss is associated with rapid proliferation and progression to advanced tumor grade in mouse models. Molecular analysis of mouse and human HCC tumors revealed that RB plays a complex role on cell cycle and other processes (e.g., adhesion and EMT) that are relevant for disease progression and associate with poor prognosis. The poor survival of HCC is due, in part, to failure to effectively treat advanced disease. Given the key role of RB in tumor etiology/progression, we investigated the ability to activate the RB-pathway as a therapeutic strategy. The findings from these studies indicated that it is possible to therapeutically reinstate the RB-pathway using specific CDK4/6 inhibitors, which are being tested in clinical trials. These combined findings provide the basis fo determining how RB modulates disease progression and how to systematically target the RB-pathway therapeutically in the treatment of HCC (Aim 2).